Systems for measuring geophysical and other parameters within and in the vicinity of a well borehole typically fall within two categorizes. The first category includes systems that measure parameters after the borehole has been drilled These systems include wireline logging, tubing conveyed long, slick line logging, production logging, permanent downhole sensing devices and other techniques known in the art. The second category includes systems that measure formation and borehole parameters while the borehole is being drilled. These systems include measurements of drilling and borehole specific parameters commonly known as “measurements-while-drilling” (MWD), measurements of parameters of earth formation penetrated by the borehole commonly known as “logging-while-drilling” (LWD), and measurements of seismic related properties known as “seismic-while-drilling” or (SWD).
For brevity, systems that measure parameters of interest while the borehole is being drilled will be referred to collectively in this disclosure as “MWD” systems. Within the scope of this disclosure, it should be understood the MWD systems also include logging-while-drilling an seismic-while-drilling systems.
An MWD system typically comprise a downhole assembly operationally attached to a downhole end of a drill string. The downhole assembly typically includes at least one sensor for measuring at least one parameter of interest, control and power elements for operating the sensor, and a downhole transmitter for transmitting sensor response to the surface of the earth for processing and analysis. Alternately, sensor response data can be stored in the downhole assembly, but these data are not available in “real time” since they can be retrieved only after the downhole assembly has been returned or “tripped’ to the surface of the earth. The downhole assembly is terminated at the lower end with a drill bit.
A rotary drilling rig is operationally attached to an upper end of the drill string. The action of the drilling rig rotates the drill string and downhole assembly thereby advancing the borehole by the action of the rotating drill bit. A receiver is positioned remote from the downhole assembly and typically in the immediate vicinity of the drilling rig. The receiver receives telemetered data from the downhole transmitter Received data is typically processed using surface equipment, and one or more parameters of interest are recorded as a function of depth within the well borehole thereby providing a “log” of the one or more parameters.
Several techniques can be used as a basis for the telemetry system. These systems include drilling fluid pressure modulation or “mud pulse” systems, acoustic systems, and electromagnetic systems.
Using a mud pulse system a downhole transmitter induces pressure pulses or other pressure modulations within the drilling fluid used in drilling the borehole. The modulations are indicative of data of interest, such as response of a sensor within the downhole assembly. These modulations are subsequently measured typically at the surface of the earth using a receiver means, and data of interest is extracted from the modulation measurements. Data transmission rates are low using mud pulse systems Furthermore, the signal to noise ratio is typically small and signal attenuation is large, especially for relatively deep boreholes.
A downhole transmitter of an acoustic telemetry induces amplitude and frequency modulated acoustic signals within the drill string. The signals are indicative of data of interest. These modulated signals are measured typically at the surface of the earth by an acoustic receiver means, and data of interest are extracted from the measurements. Once again, data transmission rates are low, the signal to noise ratio of the telemetry system is small, and signal attenuation as a function of depth within lie borehole is large.
Electromagnetic telemetry systems can employ a variety of techniques. Using one technique, electromagnetic signals are modulated to reflect data of interest. These signals are transmitted from a downhole transmitter, through intervening earth formation, and detected using an electromagnetic receiver means that is typically located at the surface of the earth. Data of interest are extracted from the detected signal. Using another electromagnetic technique, a downhole twitter creates a current within the drill string, and the current travels along the drill string. This current is typically created by imposing a voltage across a non-conducting section in the downhole assembly. The current is modulated to reflect data of interest. A voltage generated by the current is measured by a receiver means, which is typically at the surface of the earth. Again, data of interest are extracted from the measured voltage Response properties of electromagnetic telemetry systems will be discussed in subsequent sections of this disclosure.